Fuel cell power modules based on solid oxide fuel cells are well known. A solid oxide fuel cell typically combines hydrogen and oxygen to generate electric voltage and current at an anode by transport of oxygen across a solid oxide electrolyte separating a cathode in an oxygen (air) atmosphere and the anode in a hydrogen/CO atmosphere, typically reformed hydrocarbons known in the art as reformate. To gain electrical output capacity, it is known to combine a plurality of individual fuel cells into a so-called fuel cell “stack” wherein the fuel cells are connected electrically in series and are supplied and exhausted in parallel with reformate and air by respective supply and exhaust manifolds. Such a fuel cell stack is known to contain, for example, 60 individual fuel cells, which, in series, can produce approximately 42 volts at full load.
To minimize pressure and flow losses along the manifolds, as well as to provide a more compact fuel cell system, the total stack is commonly divided into two 30-cell stacks, each of which then receives separate anode and cathode gas flows in parallel although the two stacks are still connected electrically in series.
A prior art module further comprises the systems necessary for operation, including fuel supply (hydrocarbon reformate), typically including a catalytic hydrocarbon reformer; air supply for reforming, fuel cell combustion, and cooling; manifolding and ducting for anode and cathode gases; numerous valves and switches; electrical conditioning and regulating components for the module output; and an electronic controller for controlling all activities within the module.
Because of its relatively slow starting characteristics, dictated by high temperatures necessary for catalytic reforming and oxygen ion transport across the electrolyte membrane, such a fuel cell module is especially preferred for providing accessory electric power in a vehicle or a stationary application. In these applications, a fuel cell module is commonly referred to as an Auxiliary Power Unit (APU).
In some known applications, for example, in military and heavy-duty truck vehicles, the power required for the accessory load is on the order of about 5-10 kW.
In a first prior art approach, one large dedicated APU provides power for all the electrical loads and other mechanical loads such as compressors. Such a large, central system is vulnerable to overall system failure when any portion of the APU fails.
US Patent Application Publication No. 2005/0112428 A1, published May 26, 2005, and referred to herein as “the '428 publication”, discloses a second prior art approach wherein, instead of using a single large APU to meet all of the auxiliary power needs of a particular application, a plurality of smaller APU modules are provided each with a local (“slave”) controller. The local controllers are linked to a master controller to define a “fuel cell power system having multiple fuel cell modules”. In a preferred disclosed embodiment, the modules are connected in electrical series, and variations in power load are met by connecting more or fewer of the modules together. A disadvantage of such an arrangement is that the system cannot operate at a fixed voltage, as the total voltage of the system at any given time is the sum of the individual modules presently connected. As more modules are connected, the output voltage of the system increases by the added voltage of the additional modules. For many applications, for example, for automotive uses, it is highly desirable that an APU system function at a fixed voltage.
In the '428 publication, the master controller manages power production by relaying individual power production requirements to each of the slave controllers. The master controller decides what mode each module must be in and determines how much power each module must generate to contribute to the overall power requirement. Further, the master controller monitors various aspects of module and system performance to re-allocate individual power requirements amongst the power modules, as well as determining how many modules must be brought online to meet a given load.
A serious drawback of the disclosed approach is that operating efficiency as a function of load is not considered; indeed, no guidance is provided as to how the master controller is to decide how many modules should be in operation at any given time and what criteria, if any, should be used to optimize overall system performance and fuel economy.
The '428 publication discloses to remove from service and then replace any individual module that fails. A significant shortcoming of a series system is that during such removal by switching out, the system voltage is reduced and hence output power is diminished. Increasing the load on each of the remaining modules can increase the total output current but cannot restore the voltage to what it was. Further, in a series arrangement, the outright failure of any module immediately causes loss of electrical continuity, and hence failure, of the overall system.
The '428 publication teaches to load each of the modules according to its current fitness for use, and thus no thought is given to maximizing the life of each module by changing the workload allocation to balance accrued operation time among modules to balance module deterioration.
The '428 publication teaches an APU system comprising a plurality of individual fuel cell modules but is silent on geographic distribution of the modules within a particular application, for example, within a vehicle.
The '428 publication discloses that “of course, as may be necessary, any number of fuel cell power modules in the multiple fuel cell power system of the present invention can be connected in parallel as shown in FIG. 3”, but teaches no more about parallel connection and thus is silent as to how such parallel arrangement is to be controlled. No control logic is disclosed by which the master controller may achieve performance and economy objectives for such a parallel-connection system. Further, the '428 publication does not suggest or disclose to enable a variable number of modules as dictated by the load imposed on the system at any given time.
What is needed is a method for controlling an APU system comprising a plurality of individual APU fuel cell modules connected electrically in parallel to maximize the efficiency and working lifetime of the system.
It is a principal object of the present invention to provide a constant output voltage from a fuel cell APU system comprising a plurality of fuel cell modules over a wide range of power demands greater than the power output capability of an single module.
It is a further object of the invention to provide such output voltage by balancing the load among the fuel cell modules to allow each module to operate within a maximum efficiency window.